Curable silicone compositions that cure through a hydrosilylation reaction are used as protective agents or coating agents for optical semiconductor elements in optical semiconductor devices such as photocouplers, light emitting diodes, solid-state image sensing devices, and the like. In order for the above elements to emit or receive light, the cured product of such a composition must not absorb and scatter light, and furthermore, it must adhere to the above element, the lead frame, the substrate, the frame material, and the like.
For example, Patent Document 1 describes a curable silicone composition containing an organosilicon compound represented by the average unit formula:[(CH2═CH)(CH3)2SiO1/2]25(C6H5SiO3/2)75[Ep(CH3)SiO2/2]40;
Patent Document 2 describes a curable silicone composition containing an organosilicon compound represented by the average unit formula:[(CH3)(C6H5)SiO2/2]0.15[(CH3)(CH2═CH)SiO2/2]0.24(EpSiO3/2)0.19(CH3O1/2)0.42;
Patent Document 3 describes a curable silicone composition containing an organosilicon compound represented by the average unit formula:(C6H5SiO3/2)0.50(EpSiO3/2)0.25[(CH2═CH)(CH3)2SiO1/2]0.25;
Patent Document 4 describes a curable silicone composition containing an organosilicon compound represented by the average unit formula:(EpSiO3/2)0.3[(CH2═CH)(CH3)SiO2/2]0.3[(CH3)2SiO2/2]0.3(CH3O1/2)0.2; and
Patent Document 5 describes a curable silicone composition containing an organosilicon compound represented by the average unit formula:[(CH2═CH)(CH3)SiO2/2]0.2[(C6H5)2SiO2/2]0.5(EpSiO3/2)0.3. 
In the above formulas, Ep denotes a 3-glycidoxypropyl group.
However, these curable silicone compositions have the problems that light transmittance of the cured product obtained by curing is low and adhesion of the cured product to a substrate is poor.